adding PathMap to csproj breaks debugger

**OS/device including version:**
Windows 10, running the 1.0 branch with Visual Studio Community 2019

**Issue description:**
Adding the following snippet to my csproj broke the ability for visual studio to debug the program. It "hits" the breakpoints, but says "No compatible code running on the thread" and doesn't see the source file.

```

$(SolutionDir)=.

```

I'm not absolutely positive this is a bug in the extension, but it seems like a safe bet.
(My colleague ran into a similar issue using the VS Code extension and opened [this bug](https://github.com/godotengine/godot-csharp-vscode/issues/55), this report reflects the different issue that happened when I tried to do the same thing with Visual Studio.)